Polyurethane polymers are noted as having excellent abrasion resistance, good low temperature flexibility, high modulus and exceptional impact strength in many cases. Most of the polyurethane polymers presently being produced contain aromatic isocyanates, such as MDI or TDI, and are therefore sensitive to ultraviolet radiation, which restricts the use of such polymers in coatings or in molded products, particularly where prolonged outdoor use for such coating and molded products is desired, since long periods of exposure to ultraviolet light causes discoloration of such aromatic polyurethane polymers with a concurrent loss in polymer properties.
It is known that the use of ultraviolet light absorbers, antioxidants, stabilizers, and certain pigments can improve the weatherability of aromatic polyurethane polymers, thus allowing these polymers some limited uses in outdoor application. An exposure of 500 standard fade hours in a fade-o-meter without appreciable change is considered about the maximum exposure which can be obtained with aromatic polyurethane polymers with the use of various additives.
Polyurethane polymers have been prepared employing aliphatic polyisocyanates. However, the high cost of such polymers and the difficulties in manufacturing acceptable aliphatic polyurethane polymers have curtailed the use of aliphatic polyisocyanates, even though such aliphatic polyurethane polymers are far more stable to ultraviolet radiation than the aromatic polyurethane polymers.
Traditionally integrally skinned polyurethane techniques have been successful when using Reaction Injection Molding (known as RIM). However, attempts to apply and use aliphatic polyisocyanates or non-aromatic isocyanates in the RIM techniques have been faced with difficulties such as the low reactivity of the aliphatic isocyanates and the resulting poor mechanical properties of many of such aliphatic polyurethane products. It is believed that the poor mechanical properties of the aliphatic polyurethane products may be attributed to a lack of aromatic rings in the resulting polymers.
Aliphatic polyisocyanates are commercially available in many forms, such as solid materials which must be heated to become liquid for any processing reactions, and liquids which tend to be very high in viscosity, and typically must be diluted by the use of solvents, or materials which are low in viscosity, but tend to have high monomer contents and odor problems associated therewith. One available commercial aliphatic diisocyanate comprises a trimerized hexamethylene diisocyanate prepolymer which contains free NCO groups. Coatings and paints have been prepared employing a solvated trimerized hexamethylene diisocyanate with a saturated polyester resin together with metal salt catalyst. However, such coatings and paints so prepared often require days to come to a complete cure because of the slow reaction time involved.
Polyurethane elastomer moldings having a surface skin have been prepared by a reaction mixture in a reaction injection molding process by reacting polyether polymer triols and/or diols and polyesters with aromatic chain extenders, such as diethyltoluene diamine and aromatic isocyanates, such as MDI (see for example U.S. Pat. No. 4,218,543, hereby incorporated by reference). In this patent polyurethane elastomer moldings have been prepared employing aromatic diamines as chain extenders with 4, 4'-diisocyanatodiphenylmethane polyisocyanates with a polyol having a molecular weight of from about 1800 to 12,000. Reaction injecting molding requires large machines because of the fast reaction and gel times involved, while the presence of urea linkages in the resulting elastomer provides for higher heat distortion temperatures and green strengths. Most RIM parts employing the patent process require post curing and elaborate painting techniques, while the painted parts can scratch easily and chip requiring touchup work.
Solventless compositions for preparing sprayable polyurethanes and poly(urea)urethanes have been disclosed employing an isocyanate terminated prepolymer or a quasi-prepolymer prepared from aromatic isocyanates, such as TDI and MDI, as well as hexamethylene diisocyanate. The isocyanate prepolymer is reacted with a curing agent comprising as essential ingredients, a highly reactive polyol or polyamine or combinations thereof, including for example, a methylenedianiline curing agent together with a less reactive polyol filler. The reaction components are intimately admixed and sprayed through an atomizing nozzle onto a surface to produce the desired polyurethane or poly(urea)urethane coating which is essentially a solventless sprayable coating, but which takes more than 30 minutes to cure (see for example U.S. Pat. No. 4,267,299, hereby incorporated by reference).
Polyurethane coatings have also been prepared by reacting an isocyanate prepolymer, such as a prepolymer based on 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane with a polyalkylene ether polyol together with an aromatic diamine, such as the aromatic diamines described in U.S. Pat. 4,218,543. The reaction mixture produces an elastomeric polyurethane coating wich is essentially solvent-free or a low solvent coating composition.